(57) The invention relates to an improvement in bleaching of kraft pulp. In ECF bleaching
chlorine dioxide is most often a prefered bleaching chemical in certain process positions.
The disadvantage is that residual amounts of organically bound chlorine in form of
AOX, i.e organically bound chlorine in effluents, and/or OCl, i.e. chlorine organically
bound in the produced pulp, is obtained. A normal approach has been to reduce charges
of chlorine dioxide.
According to the invention could substantial reductions in AOX levels be obtained
if the process conditions in the chlorine dioxide stage elevated to above 91°C and
extended to more than 90 minutes. A major reduction of AOX up to 50% have been shown
without a corresponding increase in OCI. The chlorinated substances is degraded by
the process conditions to harmless chloride ions, instead of being liberated into
the effluent as AOX or bound to pulp as OCl.
|
|
